1. Field of the Invention
The present invention relates to a induction heating coil bobbin and more particularly to such a bobbin used to heat the hot runner in an injection molding machine at its hot runner.
2. Description of the Prior Art
There is known a so-called cold runner molding system wherein a mass of resin charged in a resin passage connecting between a molding machine nozzle and a mold cavity, that is, a so-called runner is cooled and solidified together with another mass of resin charged in the mold cavity. The solidified runner is thereafter removed together with the solidified product on mold opening. On the contrary, there is also known a hot runner type injection molding system wherein a mass of resin to be molded is cooled and solidified within the mold cavity while maintaining a runner molten. The runner in the molten state is reused in the next molding cycle.
In such a hot runner type injection molding system, a induction heating coil is used to heat and maintain the runner in its molten state, as disclosed in Japanese Laid-Open patent application No. 61-197216. The dielectric heating coil is normally mounted about a cylindrical bobbin and unitized by the use of a filler material such that the coil will not be deformed and can be easily positioned in place.
The bobbin used herein must be made of any suitable material that will not be heated under the action of magnetic flux from the induction heating coil and can withstand a raised temperature. In the prior art, ceramic materials were used to form the induction heating coil bobbins.
Since such a bobbin requires an increased accuracy in size, specialty ceramics must be used to form the induction heating coil bobbins. However, the specialty ceramics are expensive and difficult to be worked.